Satellite-Based InSAR Mapping on Turtle Mountain

What is InSAR?

Satellite-based interferometric synthetic aperture radar (InSAR) is a technique that uses repeat-pass data from polar-orbit radar satellites to map very small ground movement over relatively large areas. Those radar satellites constantly shoot beams of radar waves toward the Earth and record the time it takes the waves to reach the ground (wave phase). When the satellite revisits the same spot, the phase of the image should be identical. We combine these images to measure ground changes. From this comparison, we can view and measure ground elevation change within a few millimetres over a very broad area.

InSAR on Turtle Mountain

Initially, the area of interest to use InSAR on Turtle Mountain was the rugged, unstable, upper portion of the mountain. However, because of limitations in the orientation of available radar satellites, the technique was not considered ideal to map movement in the upper portion of the mountain, but was found to be very well suited for mapping deformations on the lower slopes. As the lower slopes and valley bottom are covered with bare-rock debris from the Frank Slide and recent rockfall events, there were thousands of points identified that provided very good quality data for deformation monitoring.

Results

By using data from Canadian Radarsat-1 from April 2004 to October 2006, Alberta Geological Survey monitored deformations over the lower slopes of Turtle Mountain and across the Frank Slide debris.
We noticed two active processes:

1. Slow movement of the loose rock was deposited on the lower slope.
2. Subsidence over abandoned underground coal mines.

Frank and Bellevue coal mines. Click on image to enlarge.

Perhaps most interesting are the ground movements associated with abandoned coal mines. Below the Frank Slide debris, the Frank and Bellevue coal mines were active in the early 1900s.

Miners made large shafts and left behind pillars of coal to support the roof. Over time, these pillars weakened, leading to slow downward movement of the roof until it collapsed, often creating a hole to the surface. In the Crowsnest Pass, many pits collapsed from this type mining.

Deformation pattern of the lower part of Frank Slide. The white stripes indicate underground coal mines; the red line outlines the Frank Slide boundary, and the black rectangle is the reference area. Click on image to enlarge.

The InSAR data have shown that the ground surface above Frank mine has been settling up to 3.15 mm annually, relative to the reference area (see figure below). Average changes of up to 3.2 mm per year, relative to the reference area (see figure below), were also observed overlying the footprint of the abandoned Bellevue underground mine to the east. As these types of abandoned coal mines are throughout Alberta, InSAR would be a valuable tool detecting and characterizing mine subsidence.